1. Field of the Invention
The present invention relates to an exposure method for semiconductor wafers and, more particularly, to block exposure.
2. Description of the Related Art
Because current LSI technology is seeking to develop larger scale integration and further miniaturized structures, it needs techniques for improving the speed and precision of exposing an LSI pattern on a semiconductor wafer. Block exposure is one of such exposure techniques.
FIG. 1 shows a conventional variable rectangular exposure unit 10. In this unit 10, a beam from a beam source 11 is irradiated on a first plate 12 and the beam, after passing a rectangular window 13 of the first plate 12, is deflected by an electromagnetic deflector 14. The cross-sectional shape of the beam which passes a rectangular window 16 formed on a second plate 15, i.e. the exposure pattern, depends on how the deflected beam is placed over the rectangular window 16. After passing the second plate the beam is further deflected by another electromagnetic deflector 17 to expose a predetermined area on a semiconductor wafer 18. This variable rectangular exposure technique cannot expose more than one pattern with a single shot of the beam.
FIG. 2 shows a conventional block exposure unit 20. In place of the second plate 15 shown in FIG. 1, this unit 20 uses a block mask 21 which has a plurality of blocks 22 each having a pattern. This block exposure can expose a plurality of patterns with a single shot of a beam. Because the patterns set on the block mask 21 affect the exposure speed and the exposure precision, it is desirable for the block mask 21 to have as many patterns as possible.
FIG. 3 is a flowchart illustrating procedures for generating data for block exposure. The block mask 21 of FIG. 2 is produced in accordance with this flowchart. In step 30 in FIG. 3, basic data (mask pattern data) from a basic data file 41 is input to a control unit (not shown) of the exposure unit 20 of FIG. 2, and various pattern processes are executed in step 31. Data collected after pattern processing is set as intermediate data in an intermediate data file 42.
Intermediate data from the intermediate data file 42 is input in step 32, and in step 33 a process of extracting a block element from the intermediate data is performed based on control statements 43. The extracted block pattern data is stored in a block pattern data file 46. For example, when the intermediate data is the pattern of memory cells 50 as shown in FIG. 4, block elements 51, 52 and 53 are extracted from the file 46 to avoid repetition.
When the intermediate data is an end portion of a cell array or a horizontally or vertically long pattern 55, as shown in FIG. 5, block elements 56, 57 and 58 are extracted from the file 46 to avoid repetition.
In step 34 of FIG. 3, the block pattern data extracted from the file 46 is received and is sent as block element data to a block data file 44. In step 35, it is determined whether the number of block elements output to the block data file 44 is equal to or greater than the number of block elements that can be installed on the block mask. The processes in steps 33-35 are repeated until the number of block elements equals or exceeds the number that can be installed.
In step 36, block pattern data is divided or segmented into rectangular shapes for exposure of the block mask based on the block data read from the block data file 44. The segmented data are output to a segment data file 45. In step 37, the segmented block data is read from the segment data file 45 and converted to exposure data which is in turn output to an exposure data file 47.
In step 38, pattern data that is different from the block elements extracted from the intermediate data file 42 is segmented into rectangular shapes, which are supplied to the segment data file 45. In step 39, segment data that is different from the extracted block elements is read from the segment data file 45 and converted to exposure data which is in turn sent to the exposure data file 47.
According to this conventional method, as shown in FIG. 4, block elements 51 to 53 extracted for fabricating the memory cells 50 are directly installed on the block mask (not shown). Also, as shown in FIG. 5, the block elements 56-58 extracted from the end portion of a cell array or the horizontally or vertically long pattern 55 according to the block size are set on the block mask (not shown) block by block even though patterns exist only at the end portion of a block. This may lead to lower exposure efficiency.
Further, the number of patterns to be installed as block elements is limited by the number of patterns that can be installed on the block mask. Therefore, those patterns which cannot be set on the block mask should be exposed by using the conventional exposure techniques. This results in a slower exposure speed and lower exposure precision.
Thus, a more efficient and faster exposure method resulting in higher precision is needed.